Xylitol is currently produced by chemical hydrogenation of xylose purified from xylan hydrolysates. The use of microorganisms to produce xylitol and other polyols from inexpensive starting materials such as corn and other agricultural byproduct and waste streams has long been thought to be able to significantly reduce production costs for these polyols as compared to chemical hydrogenation. Such a process would reduce the need for purified xylose, produce purer, easier to separate product, and be adaptable to a wide variety of raw materials from different geographic locations.
Despite a significant amount of work, development of a commercially feasible microbial production process has remained elusive for a number of reasons. To date, even with the advent of genetically engineered yeast strains, the volumetric productivity of the strains developed do not reach the levels necessary for a commercially viable process.
Xylitol is currently produced from plant materials—specifically hemicellulose hydrolysates. Different plant sources contain different percentages of cellulose, hemicellulose, and lignin making most of them unsuitable for xylitol production. Because of purity issues, only the hydrolysate from birch trees is used for xylitol production. Birch tree hydrolysate is obtained as a byproduct of the paper and pulping industry, where lignins and cellulosic components have been removed. Hydrolysis of other xylan-rich materials, such as trees, straws, corncobs, oat hulls under alkaline conditions also yields hemicellulose hydrolysate, however these hydrolysates contain many competing substrates. One of these substrates, L-arabinose is a particular problem to xylitol production because it can be converted to L-arabitol, which is practically impossible to separate from xylitol in a cost effective way.
D-xylose in the hydrolysate is converted to xylitol by catalytic reduction. This method utilizes highly specialized and expensive equipment for the high pressure (up to 50 atm) and temperature (80-140° C.) requirements as well as the use of Raney-Nickel catalyst that can introduce nickel into the final product. There have been several processes of this type described previously, for example U.S. Pat. Nos. 3,784,408, 4,066,711, 4,075,406, 4,008,285, and 3,586,537. In addition, the xylose used for the chemical reduction must be substantially purified from lignin and other cellulosic components of the hemicellulose hydrolysate to avoid production of extensive by-products during the reaction.
The availability of the purified birch tree hydrolysate starting material severely limits the xylitol industry today. If a specific, efficient reduction process could be developed that could convert xylose and arabinose to xylitol, but not reduce any other impurities that were present in a starting mixture, then a highly cost competitive process could be developed that would allow significant expansion of the xylitol market.
Many of the prior art methods of producing xylitol use purified D-xylose as a starting material and will also generally convert L-arabinose to L-arabitol (and other sugars to their respective reduced sugar polyol). While there has been a significant amount of work on the development of an organism to convert D-xylose to xylitol, none of the prior art approaches have been commercially effective. There are several reasons for this. First, D-xylose utilization is often naturally inhibited by the presence of glucose that is used as a preferred carbon source for many organisms. Second, none of the enzymes involved have been optimized to the point of being cost effective. Finally, D-xylose in its pure form is expensive. Prior art methods do not address the need for alternative starting materials. Instead they require relatively pure D-xylose. Agricultural waste streams are considered to be the most cost-effective source of xylose. These waste streams are generally mixed with a variety of other hemicellulosic sugars (L-arabinose, galactose, mannose, and glucose), which all affect xylitol production by the microbes in question. See, Walthers et al. (2001). “Model compound studies: influence of aeration and hemicellulosic sugars on xylitol production by Candida tropicalis.” Appl Biochem Biotechnol 91-93:423-35. However, if an organism can be engineered to utilize more than one of the sugars in the waste stream, it would make the process much more cost effective.
In addition to xylose, L-arabinose is an abundant sugar found in hemicellulose ranging from 5% to 20% depending on the source. Co-conversion of L-arabinose to xylitol or cell biomass would allow a greater variety of starting materials to be used (birch has very low arabinose content and thus does not lead to production of L-arabitol during the chemical hydrogenation). Therefore, methods of converting xylose and arabinose to xylitol, converting xylose and arabinose to xylitol while the arabinose remains unconverted, and converting xylose to xylitol and arabinose to biomass would be desirable.
A variety of approaches have been reported in the literature for the biological production of xylitol. While some basic research has been performed, development of an effective bioprocess for the production of xylitol has been elusive. Many of the systems described below suffer from problems such as poor strain performance, low volumetric productivity, and too broad of a substrate range. Of these, yeasts, primarily Candida, have been shown to be the best producers of xylitol from pure D-xylose. See, Hahn-Hagerdal, et al., Biochemistry and physiology of xylose fermentation by yeasts. Enzyme Microb. Technol., 1994. 16:933-943; Jeffries & Kurtzman, Strain selection, taxonomy, and genetics of xylose-fermenting yeasts. Enzyme Microb. Technol., 1994. 16:922-932; Kern, et al., Induction of aldose reductase and xylitol dehydrogenase activities in Candida tenuis CBS 4435. FEMS Microbiol Lett, 1997. 149(1):31-7; Saha & Bothast, Production of xylitol by Candida peltata. J Ind Microbiol Biotechnol, 1999. 22(6):633-636; Saha & Bothast, Microbial production of xylitol, in Fuels and Chemicals from Biomass, Saha, Editor. 1997, American Chemical Society. p. 307-319. These include Candida strains C. guilliermondii, C. tropicalis, C. peltata, C. milleri, C. shehatae, C. boidinii, and C. parapsilosis. C. guillermondii is one of the most studied organisms and has been shown to have a yield of up-to 75% (g/g) xylitol from a 300 g/l fermentation mixture of xylose. See, Saha & Bothast, Production of xylitol by Candida peltata. J Ind Microbiol Biotechnol, 1999. 22(6):633-636. C. tropicalis has also been shown to be a relatively high producer with a cell recycling system producing an 82% yield with a volumetric productivity of 5 g L−1 h−1 and a substrate concentration of 750 g/l. All of these studies however, were carried out using purified D-xylose as substrate.
Bolak Co., Ltd, of Korea describes a two-substrate fermentation with C. tropicalis ATCC 13803 using glucose for cell growth and xylose for xylitol production. The optimized fed-batch fermentation resulted in 187 g L−1 xylitol concentration, 75% g/g xylitol/xylose yield and 3.9 g xylitol L−1 H−1 volumetric productivity. See, Kim et al., Optimization of fed-batch fermentation for xylitol production by Candida tropicalis. J Ind Microbiol Biotechnol, 2002. 29(1):16-9. The range of xylose concentrations in the medium ranged from 100 to 200 g L−1 total xylose plus xylitol concentration for maximum xylitol production rate and xylitol yield. Increasing the concentrations of xylose and xylitol beyond this decreased the rate and yield of xylitol production and the specific cell growth rate, and the authors speculate that this was probably due to the increase in osmotic stress. Bolak disclosed this approach to xylitol production. See e.g., U.S. Pat. Nos. 5,998,181; 5,686,277. They describe a method of production using a novel strain of Candida tropicalis KCCM 10122 with a volumetric productivity in 3 to 5 L reactions ranging from 3.0 to 7.0 g xylitol L−1 H−1, depending on reaction conditions. They also describe a strain, Candida parapsilosis DCCM-10088, which can transform xylose to xylitol with a maximum volumetric productivity of 4.7 g xylitol L−1 H−1, again in bench scale fermentation ranging from 3 to 5 liters in size. While C. tropicalis has had moderate success in achieving relatively large levels of xylitol production than the other strains, it suffers from the fact that it is an opportunistic pathogen, and therefore is not suitable for food production and the enzyme also makes L-arabitol from L-arabinose.
One promising approach that has only been moderately explored is the creation of recombinant strains capable of producing xylitol. Xyrofin has disclosed a method involving the cloning of a xylose reductase gene from certain yeasts and transferring the gene into a Saccharomyces cerevisiae. See, U.S. Pat. No. 5,866,382. The resulting recombinant yeast is capable of reducing xylose to xylitol both in vivo and in vitro. An isolated enzyme system combining xylitol reductase with formate dehydrogenase to recycle the NADH cofactor during the reaction has been described. In this instance, the enzymatic synthesis of xylitol from xylose was carried out in a fed-batch bioreactor to produce 2.8 g/l xylitol over a 20 hour period yielding a volumetric productivity of about 0.4 g l−1 H−1. See, Neuhauser et al., A pH-controlled fed-batch process can overcome inhibition by formate in NADH-dependent enzymatic reductions using formate dehydrogenase-catalyzed coenzyme regeneration. Biotechnol Bioeng, 1998. 60(3):277-82. The use of this on a large scale using crude substrate has yet to be demonstrated and poses several technical hurdles.
Several methods for producing xylitol from xylose-rich lignocellulosic hydrolyzates through fermentative processes have been described. Xyrofin discloses a method for the production of substantially pure xylitol from an aqueous xylose solution. See, U.S. Pat. Nos. 5,081,026; 5,998,607. This solution may also contain hexoses such as glucose. The process uses a yeast strain to convert free xylose to xylitol while the free hexoses are converted to ethanol. The yeast cells are removed from the fermentation by filtration, centrifugation or other suitable methods, and ethanol is removed by evaporation or distillation. Chromatographic separation is used to for final purification. The process is not commercially viable because it requires low arabinose wood hydrolyzate to prevent L-arabitol formation and the total yield was (95 g l−1) and volumetric productivity is low (1.5 g l−1 H−1). Xyrofin also discloses a method for xylitol synthesis using a recombinant yeast (Zygosaccharomyces rouxii) to convert D-arabitol to xylitol. See, U.S. Pat. No. 5,631,150. The recombinant yeast contained genes encoding D-arbinitol dehydrogenase (E.C. 1.1.1.11) and xylitol dehydrogenase (E.C. 1.1.1.9), making them capable of producing xylitol when grown on carbon sources other than D-xylulose or D-xylose. The total yield (15 g l−1) and volumetric productivity (0.175 g l−1 H−1) coupled with the use of D-arabitol as starting material make this route highly unlikely to succeed. Additionally, a 2-step fermentation of glucose to D-arabitol followed by fermentation of D-arabitol to xylitol has also been described. See, U.S. Pat. Nos. 5,631,150; 6,303,353; 6,340,582. However, a two-step fermentation is not economically feasible.
Another method of making xylitol using yeasts with modified xylitol metabolism has been described. See, U.S. Pat. No. 6,271,007. The yeast is capable of reducing xylose and using xylose as the sole carbon source. The yeast have been genetically modified to be incapable or deficient in their expression of xylitol dehydrogenase and/or xylulose kinase activity, resulting in an accumulation of xylitol in the medium. A major problem with this method is that a major proportion of the D-xylose is consumed for growth rather than being converted to the desired product, xylitol.
A process describing the production of xylitol from D-xylulose by immobilized and washed cells of Mycobacterium smegmatis has been described. See, Izumori & Tuzaki, Production of Xylitol from D-Xylulose br Mycobacterium smegmatis. J. Ferm. Tech., 1988. 66(1):33-36. Modest titers of ˜15 g l−1 H−1 were obtained with a 70% conversion efficiency of D-xylulose into xylitol. Also disclosed was the conversion of D-xylose into xylitol by using a combination of commercially available, immobilized xylose isomerase and M. smegmatis cells containing xylitol dehydrogenase activity. It was found that xylitol inhibition of the xylose isomerase caused the incomplete conversion of D-xylose into xylitol. This process does not teach how one could relieve the inhibition of the xylose isomerase by xylitol or how one would engineer a single strain to convert D-xylose into xylitol.
Ajinomoto has several patents/patent applications concerning the biological production of xylitol. In U.S. Pat. No. 6,340,582, they claim a method for producing xylitol with a microorganism containing D-arbinitol dehydrogenase activity and D-xylulose dehydrogenase activity. This allows the organisms to convert D-arbinitol to D-xylulose and the D-xylulose to xylitol, with an added carbon source for growth. Sugiyama further develops this method in U.S. Pat. No. 6,303,353 with a list of specific species and genera that are capable of performing this transforming, including Gluconobacter and Acetobacter species. This work is furthered by the disclosure of the purified and isolated genes for two kinds of xylitol dehydrogenase from Gluconobacter oxydans and the DNA and amino acid sequences, for use in producing xylitol from D-xylulose. See, U.S. Pat. Publ. 2001/0034049; U.S. Pat. No. 6,242,228. In US Appl. Publ. No. 2003/0148482 they further claim a microorganism engineered to contain a xylitol dehydrogenase, that has an ability to supply reducing power with D-xylulose to produce xylitol, particularly in a microorganism that has an ability to convert D-arbinitol into D-xylulose.
Ajinomoto has also described methods of producing xylitol from glucose. Takeuchi et al. in U.S. Pat. No. 6,221,634 describes a method for producing either xylitol or D-xylulose from Gluconobacter, Acetobacter or Frateuria species from glucose. However, yields of xylitol were less than 1%. Mihara et al. further claim specific osmotic stress resistant Gluconobacter and Acetobacter strains for the production of xylitol and xylulose from the fermentation of glucose. See, U.S. Pat. No. 6,335,177. They report a 3% yield from a 20% glucose fermentation broth. In U.S. Pat. Appl. No. 2002/0061561, Mihara et al. claim further discovered strains, also with yields of only a few percent. See, U.S. Pat. No. 6,335,177.
Cerestar has disclosed a process of producing xylitol from a hexose such as glucose in two steps. See, U.S. Pat. No. 6,458,570. The first step is the fermentative conversion of a hexose to a pentitol, for example, glucose to arabitol, and the second step is the catalytic chemical isomerisation of the pentitol to xylitol.
Bley et al. disclose a method for the biotechnological production of xylitol using microorganisms that can metabolize xylose to xylitol. See, WO03/097848. The method comprises the following steps: a) microorganisms are modified such that oxidation of NADH by enzymes other than the xylose reductase is reduced or excluded; b) the microorganisms are cultivated in a substrate containing xylose and 10-40 grams per liter of sulphite salt (e.g. calcium hydrogen sulphite, natrium sulphite, potassium sulphite); c) the microorganisms are cultivated in an aerobic growth phase and an oxygen-limited xylitol production phase; and d) the xylitol is enriched and recovered from the substrate.
Londesborough et al. have disclosed a genetically modified fungus containing L-arabitol 4-dehydrogenase and L-xylulose xylulose reductase. See, U.S. Pat. Appl. Publ. No. 2003/0186402. This application is aimed at producing useful products from biomass containing L-arabinose, which is a major constituent of plant material but does not disclose the use of D-xylose/L-arabinose mixtures for the synthesis of xylitol in procaryotes. Verho et al. also describe and alternative L-xylulose reductase from Ambrosiozyma monospora that utilizes NADH as co-factor. See, Verho et al., New Enzyme for an in vivo and in vitro Utililization of Carbohydrates. 2004, Valtion Teknillinen Tutki-muskeskus. p. 15.
Researchers at Danisco have developed several xylitol bioprocesses. Heikkila et al. describes a process wherein purified L-xylose is utilized as intermediate. See, U.S. Pat. Appl. Publ. No. 2003/0097029. The application also covers methods of production of L-xylose. This process is not feasible because L-xylose is a rare sugar and is considerably more valuable than the final product. A method for simultaneously producing xylitol as a co-product during fermentative ethanol production, utilizing hydrolyzed lignocellulose-containing material is disclosed in U.S. Pat. Appl. Publ. No. 2003/0235881. This process consists of fermenting the free hexoses to ethanol while the xylose is converted to xylitol with a single yeast strain. The yields, however, of both ethanol and xylitol were relatively poor and require pure D-xylose as a substrate. Danisco has also developed a multiple processes for the preparation of xylitol, all of them utilizing ribulose. See, U.S. Pat. Appl. Publ. No. 2003/0125588. These processes include different conversion reactions, such as reduction, epimerization and/or isomerisation. Xylitol is also produced in the fermentation of glucose in one embodiment. The process can also use ribulose and xylulose as starting material, followed by reduction, epimerization and isomerisation to xylitol. Again the starting substrates D-xylulose and ribulose are more valuable than the final product.
Ojamo et al. shows a method for the production of xylitol involving a pair of microorganisms one having xylanolytic activity, and another capable of converting a pentose sugar to xylitol, or a single microorganism capable of both reactions. See, U.S. Pat. Appl. Publ. No. 2004/0014185. In one embodiment of the invention, two microorganisms are used for the production of xylitol, one microorganism possessing xylanolytic activity and the other possessing the enzymatic activity needed for conversion of a pentose sugar, such as D-xylose and L-arabinose, preferably D-xylose, to xylitol. This method requires a complicated two-organism system and produces mixtures of xylitol and L-arabitol, which need extra purification and recycle steps to improve the xylitol yield. It does not teach simple, single organism methods that can use D-xylose/L-arabinose mixtures to synthesize pure xylitol. Finally, Miasnikov et al. have developed multiple methods for the production of xylitol, five-carbon aldo- and keto-sugars and sugar alcohols by fermentation in recombinant hosts. See, U.S. Pat. Appl. Publ. No. 2003/0068791. These recombinant hosts have been engineered to redirect pentose phosphate pathway intermediates via ribulose-5-P, xylulose-5-P and xylitol-5-P into the production of xylitol, D-arbinitol, D-arabinose, D-xylose, ribitol, D-ribose, D-ribulose, D-xylose, and/or D-xylulose. Methods of manufacturing are disclosed that use such hosts, but the productivity is low.
While clearly there has been a significant amount of work on the development of an organism to convert xylose to xylitol, none of these have resulted in an effective production organism or a commercialized process. The yeast methods described above all require relatively pure xylose as a starting material, since the organisms described will also convert L-arabinose to L-arabitol (and other sugars to their respective reduced sugar pentitol). This results in difficult-to-remove by-products which can only be separated by costly separation methods. Purified xylose is also prohibitively expensive for use in a bioprocess and cannot compete with the current chemical hydrogenation. Several of the processes above consist of more than one fermentation step, which is again, cost-prohibitive. The reported production rate of some of the strains is low, as in the Ajinomoto patents. Above all, none of the enzymes or strains involved has been engineered to be cost effective. If the turnover rate of one or more enzyme can be improved, then the production level would increase. Further, none of the approaches have addressed the problems associated with the use of agricultural hydrolyzates to produce xylitol. Agricultural waste streams are considered to be the most cost-effective source of D-xylose. These waste streams are generally mixed with a variety of other hemicellulosic sugars (arabinose, galactose, mannose, and glucose), which all affect xylitol production by the microbes in question. See, Walthers et al., Model compound studies: influence of aeration and hemicellulosic sugars on xylitol production by Candida tropicalis. Appl Biochem Biotechnol, 2001. 91-93:423-35.
Hence there is an opportunity for a high-specificity bioprocess that is both economical and safe and can utilize alternative starting materials. Table 1 outlines several potential agricultural residues that would be suitable as feedstocks if such a process was available. The instant invention addresses these problems and allows the engineering of an efficient bioprocess for making xylitol.